High tensile strength steel having excellent press shapability

ABSTRACT

Hot rolled high tensile strength steel having excellent press shapability comprises a composition consisting of 0.03 to 0.25% of C, 0 to 0.60% of Si, 0.30 to 1.80% of Mn, less than 0.010% of S, 0.005% to 0.10% of Al, 0.005% to 0.05% of Nb, the balance of iron and impurities, and the sum of %C + 10(%S) is up to 0.25%.

Yamaguchi et a1.

HIGH TENSILE STRENGTH STEEL HAVING EXCELLENT PRESS SHAPABILITY Inventors: Tetsuo Yamaguchi; Hiroshi Kido;

Takao Nakagawa; Toshio Omori, all of c/o Nippon Kokan Kabushiki Kaisha Technical Institute 2730, Minamiwataridachio, Kawasaki-shi, Kanagawa-ken, Japan Filed: Mar. 26, 1971 Appl. No.: 128,334

Related US. Application Data Continuation-in-part of Ser. No. 764,085, Oct. 1, 1968, abandoned.

Foreign Application Priority Data 1 Oct. 23, 1973 [56] References Cited UNITED STATES PATENTS 3,155,496 11/1964 Nakamura 75/124 3,216,823 11/1965 Gulya 75/124 3,259,970 7/1966 Morita 75/124 3,328,211 7/1967 Nakamura 75/124 3,403,060 9/1968 Ito et a1. 75/124- 2,233,726 3/1941 Belding 75/124 3,402,080 9/1968 Kubuta 75/125 3,499,757 7/1970 Mandich.... 75/124 3,562,028 2/1971 Hertman 75/123 B Primary Examinerl-lyland Bizot Att0rneyFlynn & Frishauf [57] ABSTRACT Hot rolled high tensile strength steel having excellent press shapability comprises a composition consisting Oct. 5," 1967 Japan 42/63780 of 003 to 25 of C 0 to 50 f Si 0 30 130% of Mn, less than 0.010% of S, 0.005% to 0.10% of A1, US. Cl. 75/124 0 005% to 005% of Nb the balance of i and impw gf id f- 7 57l 2 :1 rities, and the sum of %C 10(%S) is up to 0.25%. ie 0 earc 7 Claims, 11 Drawing Figures I O O m A O l O 2 f I l I (SM x lo PAIENIEMM 23 ms SHEET 5 BF 5 FIGH' C O18 O19 0.0(3 OOIO Ill wZOJw IQkOz Yield Point HIGH TENSILE STRENGTH STEEL HAVING EXCELLENT PRESS SHAPABILITY RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 764,085, filed Oct. 1, 1968, now abandoned.

FIELD OF INVENTION This invention relates to hot rolled, high tensile strength steel having excellent press shapability, more particularly excellent bending shapability and workability into expansible flanges.

BACKGROUND OF THE INVENTION It is the recent trend to increase the tensile strength of steel stocks utilized in various fields of industry. In such a case it is well recognized in the art that press shapability or workability decreases with the increase in strength. For example, a similar tendency is observed in hot rolled stell plates having a thickness of about 2 to 8mm which are designated as hot rolled medium gauge plates. High tensile strength steel haying a tensile strength of niore than 50Kg]mm has a tendency of forming cracks due to working as in the case of shaping expansible flanges or bending working. Accordingly, it has been highly desired to develop high tensile strength steels of novel composition capable of performing the above described press working. However, a satisfactory steel has not yet been developed that can fullfil such requirements. Different from thin steel sheets for drawing operations, there is no report on the quantative determination of the effect of the composition or the presence of non-metal elements on the press shapability of hot rolled medium gauge steel plates.

SUMMARY OF THE INVENTION As a result of exhaustive research we have now suc-. ceeded to develop a new steel which has excellent tensile strength, productivity, and wide field of use and can be produced economically. Briefly stated, according to this invention, a definite composition is utilized for steel to improve mechanical strength and press shapability.

Thus, it is the object of this invention to improve press shapability of high tensile strength steels which have been difficult to press'work. With the novel high tensile strength steel, the weight of structural steel stocks can be reduced thus enabling sufficient decrease in weight and cost of motor cars and the like, for example.

The high tensile strength steels of excellent press shapability according to this invention comprise a composition consisting essentially of from 0.03 to 0.25 percent of C, from to 0.60 percent of Si, from 0.30 to 1.80 percent of Mn, up to 0.010 percent of S, from 0.005 to 0.10 percent of Al, from 0.005 to 0.050 percent of Nb, and the balance of Fe and impurities. The sum of %C l0(%S) is up to 0.25 percent.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawings:

FIG. 1 is a diagrammatic representation of a device utilized to test the novel steels;

FIG. 2 is a plot of the carbon content versus press shapability;

FIG. 3 is a plot of the sulfur content versus press shapability, and;

FIG. 4 is a graph to show the relationship between the phosphorus content and press shapability;

FIG. 5 is a plot of sulfur content versus numbers of charges of molten steel in the ladle;

FIG. 6 is a diagrammatic representation of a steel specimen used to determine the notch elongation thereof;

FIG. 7 is a plot of notch elongation versus press shapability;

FIG. 8 is a plot of notch elongation versus carbon content;

FIG. 9 is a plot of notch elongation versus sulfur content;

FIG. 10 is a plot of notch elongation versus 7C l0(%S) FIG. 11 is a plot of notch elongation versus yield point.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The shapability of steel stocks is usually determined by an authorized method of testing, such as those proerty of propagation of minute cracks already present in steel blanks. Accordingly, it is desirable to use a model testing device simulating the actual press working, as illustrated in FIG. 1 of the accompanying drawing. The testing device shown in FIG. 1 comprises a punch 1 and a die 2 having a letter V shaped notch of degrees at its center, the punch head being shaped to correspond to this configuration. After placing a blank 3 on the die 2, a load is applied to punch 1 to perform bending work. The characteristic value of the bending shapability is determined by the sum of the length of cracks formed at the ends of the blank 3. As a result of experiments, we have determined that this characteristicvalue well agrees with the actual shapability.

The novel high tension steels whose press shapability have been determined by the testing method described above have the following composition:

C: 0.03-0.25 percent,

Si: 0 O.60 percent,

Mn: 0.30-1.80 percent,

S: 0.010 percent Al: 0.005-0.10 percent Nb; 0.005 to 0.05 percent and the balance of iron and impurities. The sum of %C l0(%S) is up to 0.25 percent.

The novel steels of such composition can be relatively easily produced by electric furnaces, converters, or other conventional steel manufacturing furnaces so long as a suitable pretreatment of molten pig iron or a suitable steel manufacturing operation is adopted. The novel steels can be hot rolled and may be subjected to final normalizing treatment.

The range of the composition of the high tensile strength steels have excellent press shapability was determined as a result of the following experiments.

A plurality of steel samples having compositions in the above described range excepting carbon content were prepared by melting and the relationship between carbon content in the steel and press shapability was plotted as shown in FIG. 2. As can be noted from this figure, in a range in which the carbon content is less than 0.15 percent, press shapability is greatly improved as the carbon content decreases whereas it decreases rapidly above 0.25 percent. However, with carbon content of less than 0.03 percent, the effect of carbon upon high tensile strength steel was not realized. A preferred range is from 0.06 to 0.15 percent. This is the reason why the carbon content of the novel steel is limited to the above ranges.

Detailed analysis of steel plates in which cracks were formed as a result of press operation showed that silicate and alumina due to the addition of aluminum did not result in any appreciable cracks whereas sulfides resulted in cracks even though they are very minute. The relationship between sulfur content and press shapability is shown in FIG. 3 where was plotted to show a great many results of experiments made for various quantities of sulfur in the range of the composition. Significantly, the length of cracks longer than 15mm, is shown in FIG. 3. This corresponds to the sulfur content of the steel above 0.010 percent. Thus, it was found that steels containing at most 0.010 percent of sulfur could be satisfactorily used for practical application. For this reason, the sulfur content of the novel steels should be limited to 0.010 percent at the maximum and preferably should be less than 0.010 percent.

It has also be determined that there is a critical interrelationship between a total carbon and sulfur contents and excellence of press shapability. Thus, it has been found that excellent press shapability is realized when %C plus l(%S) is less than about 0.25 percent.

Industrial evaluation of press shapability of a hot rolled steel sheet is indicated by press results obtained at press works. Press shapability of a steel involves two important factors; namely, (1) deformability of a steel (when cracks are easily caused) and (2) working defects (cracks prevailing in a steel), due to the existence of artificial defects in the steel (e.g. defects caused by the existence of non-metallic materials, which is unavoidable in steel making; scratches made on the surface of steel sheets while handling the same, or defects caused by flashes when steel sheet is cut).

Commonly shapability ofa steel sheet is evaluated by elongation values obtained by a bending test or tensile test, which is a characteristic of l mentioned hereinabove. Iron and steel manufacturers often find that the results of such an evaluation do not correspond to results obtained with an industrial press. Therefore, there has been the problem of what test should be adopted to evaluate such a characteristic as mentioned in (2) hereinabove.

We have used a standard notch impact tensile test method to evaluate press shapability of hot rolled steel sheets used in the art. According to this method, press shapability is evaluated by means of elongation (GL 50mm) of tensile test pieces provided with notches and the results obtained with this method correspond to press results obtained at press works. FIG. 7 shows press results. It is true, of course, that the limit of ,causing defects in press shapability depends upon the shape of pressed products, press conditions, etc. However, in general, a steel having less than percent of notch elongation is not regarded as a steel having excellent shapability.

FIG. 8 shows the influence of carbon upon the shapability of steels having the same tensile strength with the same content of sulphur, each of which is obtained under different manufacturing conditions (e.g., coiling temperature in hot rolling). Press shapability improves remarkably with less than 0.15 percent of carbon, but it is possible to obtain a steel having excellent shapability with less than 0.25 percent of carbon by controlling sulphur content. Preferred carbon content is within the range from 0.06 percent to 0.15 percent.

FIG. 9 shows the influence of sulphur upon the shapability of steels having constant carbon content. As is clear from the data, press shapability improves remarkably with less than 0.010 percent of sulphur. The slope of the curve is substantially greater in the range of 0.004 to 0.010% S, than for values of more than 0.010% S. Solid circles in FIG. 9 represent values obtained with steels having a P content of at least 0.01 1 percent, and other circles those with up to 0.010% P.

As carbon and sulphur remarkably affect press shapability, a steel having excellent press shapability is obtained by controlling contents of carbon and sulphur. As is shown in FIG. 10, press shapability improves remarkably with less than 0.250 of (C% 10 X 8%).

FIG. 11 shows the relationship of yield point of steels of different compositions obtained under various manufacturing conditions, and press shapability. Shapability of a steel of the same composition deteriorates remarkably with a high strength value as indicated by yield point.

The effect of phosphorus is shown in FIG. 4 which was plotted from the result of experiments wherein ingredients other than phosphous were selected to be in the range mentioned hereinabove in the same manner as has been described in connection with carbon and sulfur. As can be seen from FIG. 4, in the ordinary range of composition, it is not required to take into consideration the effect of phosphorus upon the press shapability. However, the maximum limit of the phosphorus content is, preferably 0.040 percent.

As a result of our experiments it was found that, the lesser the quantity of silicon content, the more advantageous is the press working. Although silicon of trace quantity is advantageous, since silicon is incorporated as a deoxidizing agent and since it is an element to impart high tensile strength to steel at low cost, incorporation thereof to some extent should be tolerated. However, silicon content of more than 0.6 percent in steel results in increase in cost. The result of our experiments showed that the upper limit of the silicon content should be 0.6 percent, it being understood that sufficient strength can be provided for steel with silicon content less than 0.6 percent.

With regard now to manganese, when it is utilized as a deoxidizing agent it is necessary to use it in a quantity of at least 0.3 percent. However, when manganese is employed for the purpose of increasing the tensile strength of steel, this object of the invention can be achieved with a quantity of manganese of less than 1.8 percent. Incorporation of manganese in excess of 1.8 percent causes increase in the cost of the product so that the upper limit thereof was determined to be 1.8 percent.

The result of our many experiments showed that, the presence of aluminium in steel substantially improved the press shapability of the steel, but in the high tensile strength steels of this invention as the aluminium content of more than 0.10 percent has decreased the cleanness of the steel, the upper limit of aluminium was determined to be 0.10 percent. The lower limit was determined to be 0.005 percent because, with aluminium less than this limit, it is not possible to obtain the desirable deoxidizing effect of the steels. A preferred range is from 0.015 to 0.08 percent.

The novel high tensile strength steels having excellent shapability are characterized by containing niobium, in addition to various ingredients mentioned above in order to further increase the tensile strength. It was found that the effect of incorporation of this element is especially significant where steel stocks are used in the rolled state. The content of Nb was determined experimentally to range from 0.005 to 0.050 percent in due consideration of press shapability and strength of the product and it was found that this range is most suitable to attain the object of this invention.

In order to compare the novel steels with conventional steels the following examples are given.

Chemical compositions of sample steels prepared by a conventional converter are as follows:

Sample C Si Mn S Al Nb A 0.17 0.43 1.35 0.025 B 0.12 0.23 0.73 0.018 0.030 0.035 C 0.10 0.19 0.99 0.004 0.042 0.042 D 0.18 0.21 1.09 0.010 0.008 0.014 E 0.16 trace 1.14 0.008 0.041 0.013

In this table samples A and B represent controls, whereas samples C, D and E are steel stocks embodying this invention.

The results of mechanical tests made on hot rolled plates of 6mm thick of the respective samples are as follows:

.115 90V Result Tensile Bending Press of Sample strength test bending press kg/mm 1 ton, length of working 180 cracks A 55.3 good 28.0 cracked B 57.8 good 35.0 Do. C 55.3 good 0.4 No crack D 58.1 good 8.0 Do. E 56.6 good 4.5 Do.

As can be noted from this table, the novel steels of samples C to E have superior press workability over prior steels of controls A and B. It is to be particularly noted that in spite of good resultsthe JIS bending test control samples A and B produced cracks in the actual press working, whereas samples C to E did not produce any cracks. Unexpectedly, the above described data obtained by the bending test utilizing said 90V press coincided with the results of press working. As has been pointed out before, in the bending test by the 90V press, if the length of cracks were shorter than mm, there would be produced no cracks in the actual press working, all of samples C to E employed in this test showed cracks shorter than 15mm; this could be attributed to the fact that no crack was formed in the actual press working. In the above table, the results of press working for samples A, C and E were obtained with respect to the shaping of expansible flanges while those for B and D with respect to bending.

As indicated above the steels of this invention contain up to 0.010 percent sulfur, and preferably less than effective, method for desulfurizing a steel to such a sulfur content is desulfurization of molten pig iron. For example, a molten pig iron having a sulfur content of 0.046 percent is tapped from a blast furnace. The molten pig iron, 45 tons, is poured into each of two iron la dles. A desulfurizing agent 350 kg, comprising approximately 72% CaC is added to the pig iron in each ladle. Nitrogen is blown through four pipes (diameter, inch) into each ladle at a pressure of 2-5 kg/cm for 50 minutes, to agitate the molten pig iron. The contents of each ladle, approximately tons of desulfurized iron, are then poured into a ladle of a converter. A quantity (200 kg) of soda ash, serving as a desulfurizing agent, is then added to the latter ladle. Slag is removed. Samples are extracted for determining sulfur content, which is found to be 0.010 percent. The desulfurized iron is poured into a converter and is refined. The converter is tapped and ingots are then formed. Analysis of the steel product is as follows:

C Si Mn P S Nb Sol/A1 0.09 0.19 1.10 0.015 0.005 0.027 0.037

As has been described in detail, the press shapability of the novel steels is excellent so that when it is used for motor cars and the like it is possible to reduce the weight of steel utilized. Press working of a rear axle housing, for example, can be performed at high efficiencies with good yields. Of course, it should be understood that, in addition to the motor car industry, the novel steels can equally be used in any field of industry requiring high mechanical strength and excellent press workability.

Review of record has revealed that samples B E of said parent application contain Nb rather than Nb and V.

What is claimed is:

1. A high tensile strength steel having excellent pressability consisting essentially of, in percent by weight,

carbon 0.03 to 0.25

silicon to 0.60

manganese 0.30 to 1.80

sulfur to 0.010

aluminium 0.005 to 0.10

niobium 0.005 to 0.050 I and the balance iron and impurities, and the sum of %C l0(%S) being up to 0.25 percent.

2. A steel of claim 1 wherein the percent by weight of carbon is from 0.06 to 0.15.

3. A steel of claim 1 containing up to 0.040 percent phosphorus. 1

4. A steel of claim 1 containing carbon 0.09

silicon 0.19

manganese sulfur 0.005

aluminum 0.037

niobium 0.027 and phosphorus 0.015, and the said sum is 0.0905.

5. A steel of claim 1 containing carbon 0.10

silicon 0.19

manganese 0.99

sulfur 0.004

aluminum 0.042

7 8 niobium 0.042, 7. A steel of claim 1 containing and the said sum is 0.1004. carbon 16 6. A steel of claim 1 containing Silicon trace manganese l.l4 s1l1c0n 0.21 5

sulfur 0.08 manganese 1.09 sulfur 0.010 l P 0-041 aluminum 0.008 niobium niobium 0.014, and the said sum is 0.1608. and the saidsum is 0.181. 

2. A steel of claim 1 wherein the percent by weight of carbon is from 0.06 to 0.15.
 3. A steel of claim 1 containing up to 0.040 percent phosphorus.
 4. A steel of claim 1 containing carbon 0.09 silicon 0.19 manganese 1.10 sulfur 0.005 aluminum 0.037 niobium 0.027 and phosphorus 0.015, and the said sum is 0.0905.
 5. A steel of claim 1 containing carbon 0.10 silicon 0.19 manganese 0.99 sulfur 0.004 aluminum 0.042 niobium 0.042, and the said sum is 0.1004.
 6. A steel of claim 1 containing carbon 0.18 silicon 0.21 manganese 1.09 sulfur 0.010 aluminum 0.008 niobium 0.014, and the said sum is 0.181.
 7. A steel of claim 1 containing carbon 0.16 silicon trace manganese 1.14 sulfur 0.08 aluminum 0.041 niobium 0.013, and the said sum is 0.1608. 